The present invention relates to a vehicle gearbox, comprising an input shaft mounted in a housing, at least one countershaft driven by the input shaft, a main shaft, pairwise interengaging gears supported by the countershaft and the main shaft, at least one gear in each pair being freely rotatably mounted on its shaft and lockable to said shaft by means of an engaging sleeve axially displaceably but non-rotatably mounted on said shaft, said engaging sleeve being coordinated with synchronizing arrangements, a shift control, the movement of which in one direction from a neutral position effects via motion transmitting elements, displacement of an engaging sleeve to an engagement position, and the movement of which from the neutral position in another direction effects displacement of another engaging sleeve to an engagement position, and lock means comprising a lock element with control means, which, during predetermined operating conditions, hold the lock element in a locked position, in which it impedes movement of the shift control.
It is previously known to provide a gearbox of the above described type, which has the main shaft coupled to a subsequent range gearbox of planetary type, for example, in accordance with SE 9306735-5, with locking means to prevent mis-shifting. A lock function which prevents sing in the basic gearbox from the initiation of shifting in the range gearbox until this shifting step has been completed, i.e. when the shift selector mechanism has returned to its rest position, uses a sensor which cooperates with the control cylinder of the range gearbox and senses the position of its piston rod. When the driver preselects a new range (high to lower, low to high), the shifting is initiated by air being supplied to the control piston of the range gearbox, as soon as the gearbox shift lever reaches the neutral position. During initial movement of the control piston, a valve, under the influence of the sensor, directs compressed air to a control cylinder connected to a locking element. The control cylinder moves the locking element to a position in which it locks the control shaft in the neutral position. When the shifting in the range gearbox is completed, the control cylinder of the locking element is depressurized and spring means cooperating with the locking element move the locking element out of its locked engagement so that a new gear can be engaged in the basic gearbox.
Another locking function which prevents shifting from high range to low range at speeds above a predetermined speed, uses a speed sensor which controls a magnetic valve in the compressed air supply line to the control cylinder of the range gearbox. When the vehicle speed exceeds the predetermined speed limit, the magnetic valve closes and prevents compressed air supply to the control cylinder.
In gearboxes for heavy trucks, both those with and without a range gearbox with lockout function of the above described type, there are at times so-called servo-synchronizing devices. Such synchronizing devices use the rotational energy of the rotating components of the gearbox itself, to reduce the shift lever force required for the synchronizing work. In principle, shifting can occur after manual initiation of the synchronization (a light movement of the shift lever from the neutral position towards a gear speed position) completely by the action of the servo system. This involves, however, a risk. If a driver carelessly places his hand on the shift lever with the lever in the neutral position, the weight of the hand and part of the arm can be enough to move the lever to the synchronizing position without it being noticed. If the clutch pedal is depressed and the engine is disengaged from the gearbox, this will not be a problem because the shifting will be carried out normally. If, however, the clutch is not disengaged and the engine drives the input shaft of the gearbox, the synchronizing cones movable relative to each other will frictionally engage one another and soon be worn out, since it is not only the shafts, gears and discs in the gearbox which must be synchronized, but the entire engine.
The purpose of the present invention is in general to achieve a gearbox of the type described by way of introduction which is equipped so that unintended mis-shifting is made impossible, to thereby prevent initiation of the synchronizing work, especially in a gearbox with servo-synchronization, which can rapidly result in wearing out of the frictional surfaces of the synchronizing cones.
This is achieved according to the invention by virtue of the fact that the control means of the lock element comprise means which are affected by the position of a clutch pedal and which hold the lock element in the locked position when the clutch pedal is not actuated. This prevents shift lever movement from the neutral position as long as the clutch is engaged. Only after the clutch pedal has been depressed, which in practice is not done unintentionally, can the lever be moved from the neutral position for shifting. Therefore there does not have to be any risk of unconscious or unintentional initiation of the synchronizing work in a gearbox with servo-synchronization.
In a preferred embodiment of a gearbox according to the invention comprising a basic gearbox and a range gearbox and with a locking device which prevents shifting in the basic gearbox before the shifting in the range gearbox has been completed, uses a single locking device both for the locking function in the shifting in the range gearbox and for the locking function when the clutch is engaged. Instead of using, as in the previously known and described locking device, a single-acting control cylinder with spring means biasing the locking element in the direction away from the locking position, a double-acting piston cylinder device is used with spring means which bias the locking element towards the locked position. This guarantees the locking function in the neutral position even if there should occur a pressure failure in the vehicle compressed air system.